1. Field of the Invention
The present invention relates to a cruise control system for a vehicle whereby any deviation between actual vehicle speed and a target vehicle speed is controlled to a value as small as possible.
2. Description of the Related Art
Known in the prior art is a cruise control system for an automobile provided with an internal combustion engine, wherein the cruise control system has an actuator for operating an output control member of an engine, such as a throttle valve, in such a manner that the vehicle speed is controlled to a target value when the system is in a cruise mode. The cruise control system includes a sensor for detecting an actual speed of the vehicle, and means are provided for detecting a deviation of an actual vehicle speed from a target speed.
The known cruise control system has an inevitable "dead zone" where a deviation between the actual speed and the target speed is small; in this dead zone, the cruise control system does not function to eliminate the deviation. This is to prevent so-called hunting which would otherwise occur for the following reason if normal control were effected. The system is usually provided with a microcomputer system of 8 to 16 bits, and the cruise operation actuator is naturally operated by digital signals of such bits. Assuming a digital signal to the actuator is constructed of 8 bits, a number of states corresponding to 255 as indicated by a decimal number are obtained. If the output of the actuator to the control member of the throttle valve is changed between a minimum value 0 to a maximum value 100, then this value range of 0 to 100 is assigned to the 8 bit signal having 255 states. This means that the change in the value output to the actuator when the state of the least significant bit of the 8 bit signal is changed would be 100/255, which is equal to 0.39. In other words, the change in the value of the output signal to the actuator can not be smaller than 10/255=0.39, no matter how small the difference between the actual vehicle speed and the target vehicle speed. This means that, if a normal feedback control is effected at the zone having a small speed difference, the change in output value of the signal to the actuator becomes too high and hunting occurs.